In an evolution process of a Long Term Evolution (LTE) protocol, a carrier aggregation (CA) technology is introduced in Release (R) 10. In the technology, user equipment (UE) is allowed to simultaneously send or receive data on a plurality of carriers. A network device such as an evolved NodeB (eNB) selects a cell as a primary cell (Pcell) of the UE, and uses a cell on another carrier as a secondary cell (Scell) of the UE. The UE may send or receive data on a plurality of aggregated cells. Herein, an LTE system that supports CA is referred to as an LTE CA system.
Referring to FIG. 1, in the LTE CA system, each UE is corresponding to one Pcell that is a cell 1 in FIG. 1 and one or more Scells that include a cell 2 and a cell 3 in FIG. 1. Both the primary cell and the secondary cell may be used to provide a transmission resource used for uplink and/or downlink data transmission between the network device and the UE. The UE receives paging in the Pcell, and may perform a contention-based random access process in the Pcell.
At present, in a typical case, the UE in the LTE CA system supports aggregation of a maximum of five downlink carriers and aggregation of a maximum of two uplink carriers. That is, carrier aggregation of five downlink carriers and carrier aggregation of one uplink carrier are allowed, and carrier aggregation of five downlink carriers and carrier aggregation of two uplink carriers are also allowed.
FIG. 2 is a schematic diagram in which UE supports carrier aggregation of two uplink carriers and carrier aggregation of five downlink carriers in a time division duplex (TDD) LTE system. Because the UE supports two uplink carriers, the UE can send a sounding reference signal (SRS) only in an uplink subframe on the two carriers on which the UE simultaneously works. The TDD LTE system uses a time division duplex mode, and an uplink channel and a downlink channel that work on a same carrier have similar channel characteristics and are reciprocal. Therefore, if the SRS is sent only in the uplink subframe on the two carriers, only channel characteristics of downlink channels of the two carriers can be obtained, and downlink channel characteristics of remaining three carriers cannot be obtained from the SRS that is sent in the uplink subframe.